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| United States Patent |
5,302,685
|
|
Tsumura
, et al.
|
April 12, 1994
|
Method for preparing organopolysiloxane powder
Abstract
Organopolysiloxane powder is prepared from an organopolysiloxane solution
by admitting the organopolysiloxane solution into a planetary-screw mixer
and agitating the solution at a sufficient temperature to allow a
low-boiling liquid component to evaporate from the solution, thereby
separating the low-boiling liquid component from the solution.
| Inventors:
|
Tsumura; Hiroshi (Annaka, JP);
Kodana; Nobuhiko (Tomioka, JP);
Aonuma; Hidehiko (Annaka, JP);
Isobe; Kenichi (Annaka, JP)
|
| Assignee:
|
Shin-Etsu Chemical Co., Ltd. (Tokyo, JP)
|
| Appl. No.:
|
077593 |
| Filed:
|
June 17, 1993 |
Foreign Application Priority Data
| Current U.S. Class: |
528/33; 203/47; 203/48; 203/91; 264/8; 556/462 |
| Intern'l Class: |
C07F 007/08; C08G 077/06 |
| Field of Search: |
556/462
528/33
203/47
|
References Cited
U.S. Patent Documents
| 4739026 | Apr., 1988 | Riederer et al. | 556/462.
|
| 5086145 | Feb., 1992 | Morimoto et al. | 556/462.
|
| 5130400 | Jul., 1992 | Pachaly | 528/33.
|
Primary Examiner: Shaver; Paul F.
Attorney, Agent or Firm: Millen, White, Zelano & Branigan
Claims
We claim:
1. A method for preparing organopolysiloxane powder from an
organopolysiloxane solution, comprising the steps of:
admitting an organopolysiloxane solution into a generally conical hollow
tank having a vertical center axis and an inner wall, said tank being
equipped with agitating means including a rotating shaft having at least
one impeller positioned at an inclination in close proximity to the tank
inner wall, said agitating means being effective for driving said shaft
such that said shaft may rotate about its own axis and revolve about the
center axis of the tank along the tank inner wall, and
operating said agitating means for agitating the solution by rotation and
revolution of said rotating shaft at a sufficient temperature to allow a
low-boiling liquid component to evaporate from the solution, thereby
evaporating and removing the low-boiling liquid component from the
solution.
2. The method of claim 1 wherein said tank is of inverted conical shape.
3. The method of claim 1 wherein said rotating shaft has a screw shaped
impeller
4. The method of claim 1 wherein said rotating shaft has a plurality of
impellers
5. The method of claim 1 wherein said organopolysiloxane includes at least
one of units represented by the following general formulae (1) and (2):
______________________________________
SiO.sub.2
(1)
RSiO.sub.3/2
(2)
______________________________________
wherein R is independently a hydrogen atom or a substituted or
unsubstituted monovalent hydrocarbon group.
6. The method of claim 1, wherein the inclination of the rotating shaft is
about 10.degree. to 30.degree. with respect to the vertical center axis of
the tank.
7. The method of claim 5, wherein the organopolysiloxane includes at least
10 mol % of at least one of the units represented by formulae (1) and (2).
8. The method of claim 5, wherein the organopolysiloxane includes at least
30 mol % of at least one of the units represented by formulae (1) and (2).
9. The method of claim 1, wherein the organopolysiloxane becomes a solid in
the absence of the low-boiling liquid component which is removed.
Description
FIELD OF THE INVENTION
This invention relates to a method for preparing organopolysiloxane powder
by evaporating and removing a low-boiling liquid component from an
organopolysiloxane solution.
BACKGROUND OF THE INVENTION
Organopolysiloxane powder has been used as resin modifiers and paint
additives and is expected to find wider applications as elastomer
reinforcements and cosmetic additives. Organopolysiloxanes are solid in
pure form. The organopolysiloxanes, which are insoluble in solvents, for
example, have a three-dimensional molecular structure with a high degree
of polymerization, generally form with organic solvents a slurry in which
they are dispersed in liquid as solids. If the solvent is water, solid
organopolysiloxanes are afloat on water due to their water repellency.
Organopolysiloxane powder is prepared from such organopolysiloxanes by
several conventional methods including a method of separating and
collecting organopolysiloxane solids by filtration or centrifugal
separation, followed by drying and a hot air drying method of directly
drying organopolysiloxane solids through spray dryers and similar dryers.
All the conventional well-known methods for preparing organopolysiloxane
powder rely on drying. More specifically, Japanese Patent Application
Kokai (JP-A) No. 13813/1985 or U.S. Pat. No. 4,528,390 discloses
hydrolysis of a methyltrialkoxysilane in ammoniacal aqueous solution for
condensation, followed by water washing and drying. Japanese Patent
Publication (JP-B) No. 27579/1991 discloses condensation of a silane with
a polysiloxane in an aromatic heterocyclic compound solvent followed by
collection, washing and drying.
However, where organopolysiloxanes which are solid in pure form are
dissolved in a solvent to form a solution in which no solids exist,
typically when the solvent is the reaction medium which is used in
synthesizing the organopolysiloxanes, the organopolysiloxanes cannot be
separated by conventional means such as filtration and centrifugal
separation. Organopolysiloxane powder is no longer obtained.
Even in this case, organopolysiloxane powder can be yielded evaporating and
removing a low-boiling liquid component or solvent from the
organopolysiloxane solution. The use of hot air dryers such as spray
dryers provides a continuous operation which limits the residence time of
the organopolysiloxane solution within the dryer, often failing to fully
remove the low-boiling liquid component.
Batchwise operation can also be employed for evaporation. Evaporation in a
container free of an agitator requires a long time because as solids
precipitate, solid aggregates will grow, leading to a lowering of heat
transfer. Full removal of a low-boiling liquid component is not expectable
because the component can be retained in such solid aggregates. At the end
of operation, the organopolysiloxane which is now solid cannot be
discharged from the container.
Problems will arise even when evaporation is done while agitating the
organopolysiloxane solution. Conventional commonly used agitating tanks
equipped with paddle or anchor impellers allow some zones to stagnate as
solids precipitate, allowing solid aggregates to grow in the stagnant
zones as in the previous case. This results in a lowering of heat
transfer, a longer time of operation, insufficient removal of low-boiling
liquid component, and difficulty to empty the tank of the solidified
organopolysiloxane. Additionally, when the solution reaches a semi-solid
state immediately before solidification in the course of solvent
evaporation, the rotating shaft experiences increased loads. Then a
powerful motor must be used for agitation and the impeller must also be
thick and rigid enough to withstand the loads.
It is then proposed to use helical ribbon impellers which have been
conventionally used for mixing of high viscosity liquid and powder. Since
the mixing tank associated with the helical ribbon impeller is of the same
shape as the conventional agitating tanks, most of the above-mentioned
problems remain unsolved.
The loads on the impeller can be reduced by using conical blenders or
V-shaped blenders in which the tank itself rotates. These blenders,
however, are of large size, require an increased cost for installation,
and are rather inadequate for heating, evaporating and purposes.
Therefore, in a situation where organopolysiloxane solids are dissolved in
a low-boiling liquid component or solvent to form a solution, an object of
the present invention is to provide a method for preparing
organopolysiloxane powder from the organopolysiloxane solution within a
relatively short time and with a relatively reduced motive power, the
resulting organopolysiloxane powder being characterized by full removal of
the low-boiling liquid component and easy discharge from the tank.
SUMMARY OF THE INVENTION
Briefly stated, the present invention which attains the above and other
objects provides a method for preparing organopolysiloxane powder from an
organopolysiloxane solution using a conical vessel mixed by a simultaneous
rotation and revolution of a screw or an agitater (referred to as a
planetary-screw mixer) mixer whereby a low-boiling liquid component is
separated from the solution by evaporation.
More specifically, the planetary-screw mixer used herein includes a
generally conical hollow tank having a vertical center axis and an inner
wall. The tank is equipped with agitating means which includes a rotating
shaft having at least one impeller positioned at an inclination in close
proximity to the tank inner wall. The agitating means functions to drive
the shaft such that the shaft may rotate about its own axis and revolve
about the center axis of the tank along the tank inner wall. The agitating
means is operated for agitating the solution by rotation and revolution of
the rotating shaft at a sufficient temperature to allow a low-boiling
liquid component to evaporate from the solution, thereby evaporating and
removing the low-boiling liquid component from the solution and yielding
the organopolysiloxane powder.
In accordance with the method of the invention, organopolysiloxane powder
which is fully free of the low-boiling liquid component is obtained within
a relatively short time while requiring a relatively reduced motive power.
The resulting organopolysiloxane powder is substantially free of
aggregates and substantially free of the low-boiling liquid component and
is thus sufficiently free flowing to discharge from the tank. The
invention is particularly advantageous for obtaining in powder form an
organopolysiloxane which contains a certain amount of a Q unit represented
by SiO.sub.2 and/or a T unit represented by RSiO.sub.3/2 and forms a solid
in the absence of a low-boiling liquid component, typically solvent.
In general, organopolysiloxanes are comprised of at least one unit selected
from the group consisting of
(1) a unit represented by SiO.sub.2 (referred to as a Q unit),
(2) a unit represented by RSiO.sub.3/2 wherein R is independently a
hydrogen atom or a substituted or unsubstituted monovalent hydrocarbon
group having 1 to 12 carbon atoms (referred to as a T unit),
(3) a unit represented by R.sub.2 SiO wherein R is as defined above
(referred to as a D unit), and
(4) a unit represented by R.sub.3 SiO.sub.1/2 wherein R is as defined above
(referred to as a M unit). Among them, those organopolysiloxanes
containing a certain amount or more of at least one of the Q and T units
become solid in the absence of a low-boiling liquid component, typically
solvent. For example, organopolysiloxanes consisting of M and Q units
wherein R is methyl change as follows in accordance with the M/Q unit
molar ratio.
##EQU1##
Those organopolysiloxanes containing a M and/or D unit as a basic
constituent unit are generally liquid or elastomeric at room temperature.
Since the removal of low-boiling liquid component by evaporation in
conventional agitating tanks is difficult to apply to those
organopolysiloxanes which become solid as the low-boiling liquid component
is removed by evaporation, the prior art procedure is limited to those
organopolysiloxanes containing a M and/or D unit as a basic constituent
unit and those organopolysiloxanes containing Q and/or T unit which melt
and fluidize upon heating.
In accordance with the method of the invention adapted to subject an
organopolysiloxane solution to evaporation in a specific agitating tank,
even those organopolysiloxanes which contain a Q and/or T unit as a basic
constituent unit and become solid as the low-boiling liquid component is
removed by evaporation can be subjected to evaporation for removing the
low-boiling liquid component therefrom. Furthermore, even if solids
precipitate during evaporation, the rotation of the rotating shaft
(inclusive of rotation about its own axis and revolution about the tank
axis) provides grinding of solid precipitates at the same time as solids
precipitate. The concurrent solid precipitation and pulverization allows
the solvent removal-by-evaporation procedure to continue without growing
solid aggregates. Then the low-boiling liquid component can be fully
removed while maintaining good thermal conduction throughout the tank.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic vertical cross-sectional view of one exemplary
agitating tank used in the practice of the invention.
FIG. 2 schematically illustrates an overall system for practicing the
method of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In accordance with the method of the present invention, organopolysiloxane
powder is prepared from an organopolysiloxane solution in a tank by
evaporating and removing a low-boiling liquid component therefrom. The
tank used is a generally conical or frustoconical hollow tank having a
vertical center axis and an inner wall. The tank is equipped with
agitating means which includes a rotating shaft having at least one
impeller positioned at an inclination in close proximity to the tank inner
wall. The impeller may be a screw-shaped impeller or a plurality of
impellers. The agitating means functions to drive the shaft such that the
shaft may rotate about its own axis and revolve about the center axis of
the tank along the tank inner wall. After the organopolysiloxane solution
is admitted into the tank, the agitating means is operated for agitating
the solution by rotation and revolution of the rotating shaft and impeller
at a sufficient temperature to allow a low-boiling liquid component to
evaporate from the solution, thereby evaporating and removing the
low-boiling liquid component from the solution and yielding the
organopolysiloxane powder.
Referring to FIG. 1, there is illustrated one preferred embodiment of the
agitating tank used herein. Illustrated is a fixed vessel type conical
screw mixing tank equipped with a screw agitator which is known as a
single arm planetary-screw mixer.
In FIG. 1, an inverted conical or frustoconical hollow tank 1 has a
vertical center axis (not shown) and a conical or frustoconical inner
wall. The tank 1 includes a vent 2 at its top and a discharge port 3 at
its bottom. A heating jacket 9 surrounds the tank. The tank 1 can contain
an organopolysiloxane solution. The tank 1 is equipped with an agitator 4
including a center shaft 5 in alignment with the tank center axis, an arm
6 coupled to the center shaft 5 for revolution therewith, and a rotating
shaft 8 rotatably connected to the art 6 at its distal end. The rotating
shaft 8 has a screw impeller 7 spirally extending around the shaft. The
rotating shaft 8 extends between the arm end and the tank bottom and
parallel to the tank inner wall with a close spacing therefrom. The
rotating shaft 8 has an inclination in conformity to the conical inner
wall. The center shaft 5 revolves together with the arm 6 and as a result,
the rotating shaft 8 revolves about the tank center axis making a circular
movement along the tank inner wall. Separately, the rotating shaft 8 is
rotatable about its own axis. For the driving purpose, a motor assembly 10
is mounted at the top of the center shaft 5. The motor assembly 10
provides a motive force for driving the agitator for providing double
actions including revolution of the rotating shaft 8 (or screw impeller 7)
about the tank center axis and rotation of the rotating shaft 8 (or screw
impeller 7) about its own axis.
The planetary-screw mixer of this type is commercially available as SV
mixer from Shinko Pantech K. K. and Nauta mixer from Hosokawa Micron K. K.
Preferably the inclination of the tank inner wall or rotating shaft 8 is
about 10.degree. to 30.degree. with respect to the vertical axis.
Inclination in excess of 30.degree. or gentle slope is less desirable in
discharging the organopolysiloxane powder from the tank 1 through the
discharge port 3 because some powder can remain in the tank. A tank with
an inclination angle of less than 10.degree. is vertically elongated and
will undesirably require a space of an increase height for installation.
According to the invention, an organopolysiloxane solution is admitted into
a planetary-screw mixer as mentioned above and agitated at elevated
temperature by double actions of the screw impeller including rotation
about its own axis and revolution about the tank axis for removing a
low-boiling liquid component from the solution by evaporation. The
organopolysiloxane used herein must become solid upon removal of a
low-boiling liquid component by evaporation. More particularly, the method
of the invention is advantageously applied to an organopolysiloxane which
contains at least 10 mol %, especially at least 30 mol % of at least one
of Q and T units, especially both and which becomes solid in the absence
of a low-boiling liquid component, typically organic solvent. The
remaining unit may be D unit and/or M unit.
The method of the invention is also applicable to an organopolysiloxane
which remains liquid upon removal of a low-boiling liquid component by
evaporation, for removing the low-boiling liquid component therefrom by
evaporation. The method of the invention is especially advantageous to
those organopolysiloxanes which have a higher molecular weight and a
higher viscosity. Additionally, the invention is applicable to a slurry
that organopolysiloxane powder forms with a non-solvent and a suspension
in which organopolysiloxane powder is afloat on liquid because the powder
can be recovered by evaporating and removing the liquid component. The
invention is particularly advantageous when applied to fine powder having
a mean particle size of less than 10 .mu.m or powder which is difficult to
separate by filtration and where more precise drying than the continuous
drying as achievable by spray dryers is desired.
The low-boiling liquid component present in the organopolysiloxane solution
is typically the solvent used in the synthesis of the organopolysiloxane,
for example, aromatic hydrocarbons such as benzene, toluene and xylene,
alcohols such as methanol, ethanol and propanol, ketones such as methyl
ethyl ketone and methyl butyl ketone, and low molecular weight siloxanes
such as hexamethyldisiloxane and octamethylcyclotetrasiloxane. The
invention is applicable to any low-boiling liquid component having a
boiling point which is sufficiently different from the intended
organopolysiloxane to separate therefrom by simple distillation.
For obtaining organopolysiloxane powder, a system as shown in FIG. 2 may be
used. The system includes the agitating tank 1 of FIG. 1 wherein an
organopolysiloxane solution is heated and agitated while a low-boiling
liquid component is separated therefrom by evaporation.
In operating the rotating shaft 8 or impeller 7 for providing agitation,
the rotation of the shaft or impeller about its own axis is preferably 30
rpm or more, especially 40 to 150 rpm and the revolution of the shaft or
impeller about the tank axis is preferably 1/2 rpm or more, especially 1
to 5 rpm, both for accomplishing sufficient mixing.
The tank 1 may be heated by feeding a hot medium, typically steam to the
jacket 9 from a supply conduit 11, the medium being recovered through a
return conduit 12. The heating medium through the jacket 9 should have a
temperature above the boiling point of the low-boiling liquid component
under the pressure prevailing in the tank. An electric heater may be used
for heating instead of the heating jacket.
As a result of the heating/agitating procedure, the low-boiling liquid
component will evaporate and escape from the tank 1 through the vent 2.
The vapor is then channeled through a discharge conduit 13 and condensed
in a condenser 14. The condenser 14 is cooled by feeding a coolant such as
brine from a supply conduit 15 and returning it to a return conduit 16.
The low-boiling liquid component which is condensed in the condenser 14 is
collected in a reservoir 17. The reservoir 17 is also preferably cooled as
is the condenser 14 because re-evaporation of the low-boiling liquid
component therein is prevented. When the low-boiling liquid component
vapor is discharged from the tank 1, powdered organopolysiloxane is
scattered and entrained by the vapor and conveyed there-with into the
discharge conduit 13, causing a lowering of the powder yield. For
collecting such vapor-borne powder, a powder collector 18 in the form of a
bag filter or cyclone is preferably disposed in the discharge conduit 13
between the vent 2 and the condenser 14. The collector 18 is a bag filter
in the illustrated embodiment wherein compressed inert gas such as
nitrogen gas is fed in pulses from a gas feed line 19 to the bag filter
for back washing, thereby clearing off the powder and preventing the
filter from clogging.
Evaporation of the low-boiling liquid component can be promoted by keeping
the system in vacuum. To this end, a vacuum generator 20 in the form of a
vacuum pump or steam ejector may be connected to the discharge conduit.
Vacuum is produced in the tank 1 such that the boiling point of the
low-boiling liquid component at that vacuum is lower than the temperature
of the heating medium, preferably for establishing a difference of
50.degree. C. or more between the boiling point of the low-boiling liquid
component and the temperature of the heating medium.
As the low-boiling liquid component evaporates and separates from the
organopolysiloxane solution in the tank, the solution increases in
viscosity and sometimes tends to bubble. In the latter case, the
temperature or vacuum may be properly controlled so as to prevent bumping.
The thus obtained organopolysiloxane powder is discharged from the tank 1
by gravity simply by opening a valve associated with the discharge port 3.
A ball valve and plug valve are exemplary valves. A discharge controller
in the form of a rotary valve may be provided where it is desired to
control the discharge rate. Simply by locating the discharge port 3 at the
bottom of the tank 1, the tank can be readily emptied of the powder.
EXAMPLE
Examples of the invention are given below by way of illustration and not by
way of limitation.
EXAMPLE 1
A planetary-screw mixer (fixed tank type conical screw mixer, available as
a SV mixer from Shinko Pantech K.K.) as shown in FIG. 1 was used. The
mixer had an effective tank volume of 200 liters and was equipped with a
motor of 5.5 kW for screw spinning and a motor of 0.2 kW for screw
revolution. The screw was disposed parallel to the inner wall at an
inclination angle of 17.degree. relative to the vertical axis. The tank
was charged with 100 kg of a siloxane resin solution consisting of 70% by
weight of a siloxane resin having a M/Q unit molar ratio of 0.85 (M unit:
(CH.sub.3).sub.3 SiO.sub..kappa. unit, Q unit: SiO.sub.2 unit) and 30% by
weight of toluene. The solution was a clear colorless solution having a
viscosity of 40 centipoise.
The mixer was installed in a system as shown in FIG. 2. The screw was
rotated at 60 rpm about its own axis and revolved at 2 rpm in a planetary
orbit about the tank axis. Heating was started by feeding steam at 4
kg/cm.sup.2 G to the jacket. When the temperature in the mixing tank
reached 60.degree. C., the vacuum pump was actuated to start evacuation of
the tank interior whereupon toluene started distilling out. Toluene vapor
was condensed in a condenser for recovery.
With the mixing tank continuously heated, while observing bubbling in the
tank, the degree of vacuum was gradually lowered such that no bumping
occurred. The solution in the tank gradually increased its viscosity. When
a pressure of 30 Torr and a temperature of 80.degree. C. were reached in
the tank, the contents in the tank turned solid. The powder which was
scattered and conveyed together with the toluene vapor flow was collected
by the bag filter disposed midway the vapor discharge conduit while the
system continued to operate. The bag filter was cleaned every two minutes
by reversely feeding nitrogen gas in pulses. Although aggregates were
found at the initial point of time when the contents turned solid, the
aggregates were ground as the operation continued, the contents resulting
in white powder. The motors could continue operation for agitation without
overload.
When a pressure of 5 Torr and a temperature of 130.degree. C. were reached
in the mixing tank, heating was interrupted and nitrogen gas was admitted
into the tank interior until atmospheric pressure was resumed. The
duration from the start to the end of heating was 41/3 hours.
The valve at the tank bottom was opened to discharge the resulting powder.
Simply by fully opening the valve, the tank could be completely emptied of
the powder except for some deposits.
The powder was white and had a weight of 67 kg and a yield of 95.7%. A
portion of the powder was heated in a dryer at 105.degree. C. for 3 hours
to determine a heat loss of 0.006% by weight.
COMPARATIVE EXAMPLE 1
An anchor agitator tank (vertical mixing tank plus anchor impeller) having
an effective volume of 200 liters and equipped with a motor of 5.5 kW and
an anchor impeller was charged with the same siloxane resin solution as in
Example 1. The solution was agitated at 60 rpm. Heating was started by
feeding steam at 4 kg/cm.sup.2 G to the jacket. When the temperature in
the mixing tank reached 60.degree. C., the vacuum pump was actuated to
start evacuation of the tank interior whereupon toluene started distilling
out. Toluene vapor was condensed in a condenser for recovery.
With the mixing tank continuously heated, while observing bubbling in the
tank, the degree of vacuum was gradually lowered such that no bumping
occurred. The solution in the tank gradually increased its viscosity. When
a pressure of 30 Torr and a temperature of 80.degree. C. were reached in
the tank, the motor stopped due to overloading.
With the motor stopped, the powder which was scattered and conveyed
together with the toluene vapor flow was collected by the bag filter
disposed midway the vapor discharge conduit while the system continued to
operate. The contents turned solid, but formed a mass interfering with the
agitating impeller against rotation. When a pressure of 5 Torr and a
temperature of 130.degree. C. were reached in the mixing tank, heating was
interrupted and nitrogen gas was admitted into the tank interior until
atmospheric pressure was resumed. The duration from the start to the end
of heating was 8.5 hours.
The valve at the tank bottom was opened, but little of the solid discharged
out. The mass was manually broken into fragments which fell down through
the valve.
The solid product contained a multiplicity of aggregates in powder. Its
weight was 52 kg with a yield of 74.3%. A portion of the powder was heated
in a dryer at 105.degree. C. for 3 hours to determine a heat loss of 0.2%
by weight.
COMPARATIVE EXAMPLE 2
Using a spray dryer equipped with an electric heater having a power of 15
kW and a rotary disk atomizer, the same siloxane solution as used in
Example 1 was sprayed at a rate of 22 kg/hour while feeding nitrogen gas
heated at 150.degree. C. White powder collected in a receiver below the
spray dryer and was continuously taken out of the receiver. The system was
operated for 4.5 hours until a throughput of 100 kg was reached. At this
point, feed of the siloxane solution was stopped and the operation was
interrupted.
The thus collected powder, combined with the powder collected in the
cyclone, had a weight of 64 kg and a yield of 91.4%. A portion of the
powder was heated in a dryer at 105.degree. C. for 3 hours to determine a
heat loss of 0.3% by weight.
As compared with Comparative Examples 1 and 2, Example 1 using a
planetary-screw mixer produced a powder having a minimized residue of
low-boiling liquid component. Example 1 had the advantages of elimination
of motor interruption by overloading, a short processing time, and easy
powder discharge from the tank as compared with Comparative Example 1.
These results are shown in Table 1.
TABLE 1
______________________________________
Comparative
Comparative
Example 1
Example 1 Example 2
______________________________________
Apparatus planetary- anchor spray dryer
screw mixer
agitator
Raw material
70 wt % siloxane value
(M/Q = 0.85) + wt % toluene
Evaporating time
41/3 hours 8.5 hours 4.5 hours
Motor operation
good interrupted
--
Residual low-
0.006% 0.2% 0.3%
boiling liquid
component
Discharge from
easy difficult easy
tank
______________________________________
EXAMPLE 2
A planetary-screw mixer as used in Example 1 was charged with 100 kg of a
siloxane resin solution consisting of 50% by weight of a siloxane resin
consisting of 100% T units which were (CH.sub.3)SiO.sub.3/2 units and 50%
by weight of toluene. The solution was a clear colorless solution having a
viscosity of 12 centipoise.
The screw was rotated at 60 rpm about its own axis and revolved at 2 rpm
about the tank axis. Heating was started by feeding steam at 100.degree.
C. to the jacket. When the temperature in the mixing tank reached
40.degree. C., the vacuum pump was actuated to start evacuation of the
tank interior whereupon toluene started distilling out. Toluene vapor was
condensed in a condenser for recovery.
With the mixing tank kept at 40.degree. C., while observing bubbling in the
tank, the degree of vacuum was gradually lowered such that no bumping
occurred. The solution in the mixing tank gradually increased in
viscosity. When the pressure in the mixing tank lowered to 30 Torr, the
contents in the tank turned solid. The powder which was scattered and
conveyed together with the toluene vapor flow was collected as in Example
1 while the system continued to operated. Although aggregates were found
at the initial point of time when the contents turned solid, the
aggregates were ground as the operation continued, the contents resulting
in white powder. The motors could continue agitating operation without
overload.
When a pressure of 5 Torr was reached in the mixing tank, heating was
interrupted and nitrogen gas was admitted into the tank interior until
atmospheric pressure was resumed. The duration from the start to the end
of heating was 6.5 hours.
The valve at the tank bottom was opened to discharge the resulting powder.
Simply by fully opening the valve, the tank could be completely emptied of
the powder except for some deposits.
The powder was white and had a weight of 47 kg and a yield of 94.0%. A
portion of the powder was heated in a dryer at 105.degree. C. for 3 hours
to determine a heat loss of 0.05% by weight.
It is evident from Examples 1 and 2 that organopolysiloxane powder can be
produced whether the major units of organopolysiloxane are M and Q units
or T units.
There has been described a method for producing free-flowing
organopolysiloxane powder having a minimized content of a low-boiling
liquid component within a relatively short time while requiring a
relatively reduced motive power. The organopolysiloxane powder is
characterized by a minimized content of a low-boiling liquid component and
free flow or efficient removal from the tank.
Although some preferred embodiments have been described, many modifications
and variations may be made thereto in the light of the above teachings. It
is therefore to be understood that within the scope of the appended
claims, the invention may be practiced otherwise than as specifically
described.
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